1. Collect intelligence - determine the command and control structures, steal new technologies (unlikely we'd have anything new to the alien)
2. Engineer a supervirus to kill a large proportion of the human population
3. Attach densely populated areas with nuclear weapons (nuclear weapons would be used sparingly to avoid contamination of the planet)
4. Deploy robots or remote controlled vehicles to kill the remaining human beings
5. Land and harvest the resources
Another potential option would be engineering a third world war, however this would probably have a lower casulty rate than a supervirus and would risk contamination of the planet with nuclear fallout.
Unlike most alien fiction, the aliens would not do any fighting, or sustain any losses (asymmetric warfare). If we're sufficiently advanced now that we can use Unmanned Aerial Vehicles (UAVs) in Iraq and Afghanistan, then it's likely that any alien's would also use (more advanced) robots.